The e + e − → μ + μ − reaction has been studied at centre of mass energies ranging between 38.3 abd 46.8 GeV with the CELLO detector at PETRA. We present results on the cross section and the charge asymmetry for this channel. Combining all the data at the average energy 〈 s 〉=43 GeV we obtain R μμ =〈 σ μμ / σ 0 〉=0.98±0.04±0.04, 〈 A μμ 〉=(−14.1±3.7±1.0)%, where σ 0 is the QED cross section and A μμ is the charge asymmetry corrected for pure radiative effects. These results are in good agreement with the expected values of R μμ =1.01 and A μμ =−14.5% at that energy.
Mu-pair cross sections.
Corrected angular distributions with data sample divided into two energy regions with means 39 and 44 GeV and total energy region.
Forward-backward asymmetry.
A high-precision measurement of the differential cross section for Bhabha scattering (e+e−→e+e−) is presented. The measurement was performed with the MAC detector at the PEP storage ring of the Stanford Linear Accelerator Center, at a center-of-mass energy of 29 GeV. Effects due to electroweak interference are observed and agree well with the predictions of the Glashow-Salam-Weinberg model. The agreement between the data and the electroweak prediction rules out substructure of the electron up to mass scales of 1 TeV.
Error contains both statistics and systematics.
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High-precision measurements of electron-positron annihilation into final states of two, three, and four photons are presented. The data were obtained with the MAC detector at the PEP storage ring of the Stanford Linear Accelerator Center, at a center-of-mass energy of 29 GeV. The measured e+e−→γγ differential cross section is used to test the validity of quantum electrodynamics (QED) in this energy range; it agrees well with QED, and the limit on cutoff parameters for the electron propagator is Λ>66 GeV. The measurement of e+e−→γγγ is used to test the QED calculations of order α3 and to search for anomalies that would indicate the existence of new particles; the agreement with QED is excellent and no anomalies are found. Two events from the reaction e+e−→γγγγ are found, in agreement with the QED prediction.
Errors are combined statistical and systematics.
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Two 4gamma events are observed corresponding to a cross section of 0.02 PB.
We have studied 419 τ pair events produced in the reactione+e−→τ+ τ− at a c.m. energy of 34.6 GeV. We measure the cross section and angular distribution, as well as the decay branching ratios. The production characteristics are consistent with the Standard Electroweak Model predictions of γ andZ0 interference. The branching ratios are generally consistent with the τ decaying according to standard weak interaction principles, but we observe somewhat more decays resulting in single charged hadrons plus neutrals than are predicted by present theory.
Corrected for radiative effects.
Measured cross section relative to Standard Model Prediction.
Asymmetry based on fits to angular distribution.
The differential cross section of the reactione+e−→e+e− at a c.m. energy of 34.7 GeV has been measured. The result, together with our previously measurede+e−→α+α− data, are compared with the standard model predictions. We obtain for the weak neutral current couplings the valuesgv2=0.09×0.06,ga2=0.38×0.08. A fit of the Weinberg mixing angle gives the valuegv2=0.09×0.06,ga2=0.038×0.08. The data are also used to set limits on possible deviations from the pointlike structure of leptons. An upper limit for thee+e− coupling to a heavy spin 0 boson is also given.
Fully corrected results for Bhabha scattering.
The differential cross section for Bhabha scattering.
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We have studied the reactions e + e − → e + e − , e + e − → γγ , e + e − → μ + μ − , and e + e − → τ + τ − in the centre-of-mass (CM) energy range from 39.8 to 45.2 GeV using the CELLO detector at PETRA. Upper limits on the partial widths for new spin 0 bosons with masses both within and above the energy range covered are determined. No evidence for contributions of such new particles has been observed up to the highest PETRA energies in a model independent way. Under the assumptions of recently suggested models relating the existence of spin 0 bosons to the radiative width Γ τ of the Z 0 we exclude such bosons at the 95% confidence level for masses below the Z 0 -mass if Γ τ > 20 MeV.
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Figure actually gives the 95 PCT CL upper limits of the coupling constants for each process as a function of the mass of the intermediate spin zero boson.
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The total and differential cross sections of the K¯0p→Λπ+ and K¯0p→∑0π+ reactions have been measured in the centre-of-mass energy range of l.5 to 2.3 GeV. Using our K¯0p→∑0π+ data as well as available cross-section data of isospin related channels, we have calculated the total I=0K¯N→∑π cross section as function of energy. The results are compared with predictions obtained from K¯N phase-shift analyses.
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